1. Field of the Invention
The present invention relates to a display panel, and more particularly, an organic electroluminescent (hereinafter referred to as EL) display panel.
2. Background of the Related Art
The technology of organic EL devices, also called organic light emitting diodes (LEDs), has been rapidly advancing, and some prototype modules have been demonstrated at exhibitions. An example of an organic EL device is described in U.S. Pat. No. 5,701,055. Organic EL devices are extremely thin, matrix-addressable and operable at a relatively low voltage, typically less than 15 volts. Furthermore, they have additional features suitable for the next generation of flat panel displays (FPDs) such as, for example, little dependence on viewing angle and good device-formability on flexible substrates. One of the drawbacks of liquid crystal displays is that most of them require bright backlighting, which can be easily eliminated by the use of an organic EL display.
Organic LEDs differ fundamentally from conventional inorganic LEDs. While the charge transfer in inorganics is band-like in nature and the electron-hole recombination results in the interband emission of light, organic films are generally characterized by the low-mobility activated hopping transport and the emission is excitonic. Organic EL devices are also substantially different from conventional inorganic EL devices, especially in that organic EL devices are operable at low DC voltages.
A substantial amount of effort has been directed towards the efficiency improvement and color control of organic LEDs. Further, for commercialization and manufacturing of an organic EL device for various products and applications, considerations of manufacturability, uniformity, reliability, and systems issues is quite important. Moreover, for the applications to large flat panel displays, it is critical to have uniform emission over the whole display screen. Another important issue indirectly but closely associated with the fabrication of a large organic EL display panel is the difficulties associated with driving a large organic EL panel using a passive matrix addressing scheme. One way to drive an organic EL panel is to have organic function layers sandwiched between two sets of orthogonal electrodes, i.e. rows and columns. In this passive addressing scheme, the EL element serves both the display and switching functions. The diode-like nonlinear current-voltage characteristic of the organic EL element should, in principle, permit a high degree of multiplexing in this mode of addressing. However, the problems to be discussed below become apparent as an organic EL display panel becomes large.
First, it is well known that an RC time delay is significant in an organic EL display due to somewhat poor conductivity of indium tin oxide (ITO), a typical material for transparent electrode, and relatively large capacitance component of organic layers. As the panel size increases, the time delay becomes prohibitively large.
Further, an organic EL display element does not have an intrinsic memory requiring a very high peak luminance for a passive matrix addressing, which limits the number of rows of the display panel. The instantaneous peak luminance is proportional to (number of row) x (average luminance). To achieve an average display luminance of 100 cd/m.sup.2, for example, the maximum number of rows will probably be limited to less than 500. The estimation is made with the assumption that a peak luminance is 50,000 cd/m.sup.2, which is not trivial to achieve, and the device stability will certainly be a critical issue at that high luminance. In addition, the instantaneously high current causes large IR potential drops along the column and row buses, which causes the non-uniformity of brightness over the panel surface.
One solution to the problems discussed above may be an active addressing scheme as in a thin film transistor liquid crystal display (TFT-LCD). But an active matrix EL (AM-EL) is costly to fabricate, which makes the organic EL panel less competitive compared to other display technologies, e.g., plasma display panel (PDP), as the display size becomes large. Thus, it is highly in demand to devise a practical way to fabricate a large passive addressable organic EL panel.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.